1. Field of the Invention
The present invention relates to an optical system that uses Raman pump wave. Typical examples of the optical system include an optical amplifier; a Raman pump wave emitting device, for example, a Raman pump wave emitting device for optical amplification by stimulated Raman effect in an optical fiber; and an optical repeater; or an optical system in which those devices are used.
2. Related Arts
Wavelength division multiplexing (WDM) optical-transmission method, by which a plurality of optical signals having a different wavelength one another are multiplexed in an optical fiber to transmit information, is a very effective method for increasing capacity of optical fiber communication. For the purpose of repeating/amplifying such optical signals, the following optical fiber amplifiers are used: an optical fiber amplifier doped with rare-earth elements such as EDFA (Erbium-doped Fiber Amplifier); a semiconductor optical amplifier; and a Raman amplifier. Those optical amplifiers are used as a post-amplifier that is located on an upstream side of an optical fiber transmission line, and that amplifies a transmission optical signal with one wavelength or a plurality of wavelengths to obtain a large optical power for outputting. In addition, the optical amplifiers are widely used as the following: a preamplifier that is located on a downstream side of an optical fiber transmission line, and that amplifies a attenuated optical signal after transmission to receive the signal; an optical repeater that amplifies an optical signal after transmission to forward the signal to the next optical fiber transmission line; and the like.
Among those optical amplifiers, in particular EDFA that has already become widespread in optical fiber transmission systems for commercial use is equipped with an interruption circuit that detects a loss of an input signal and open of an output end (such as connector's open and fiber's cut) to interrupt an output of the optical amplifier. A method for detecting a loss of an input signal to interrupt an optical amplifier is described in Japanese Patent Application Laid-Open No. Hei 7-240717 (Literature 1) for example. A method for detecting an output connector's open to interrupt an optical amplifier is described in Japanese Patent Application Laid-Open No. Hei 07-190887 (Literature 2) for example, and others.
FIG. 2 illustrates a configuration of conventional optical amplifier 100 having an interrupting mechanism by detecting a loss of input light and output's open. The optical amplifier 100 amplifies an optical signal, which has been input from an input light connector 101, using an optical amplification medium 103 such as EDF (Erbium Doped Fiber) to output the signal from an output light connector 105. During operation of normal optical amplification, the optical amplification medium is exited by pump wave that is output from an pump wave source 111. As a result, a gain of the optical signal is provided. Generally, the optical amplifier 100 is controlled so that output intensity of pump wave is kept constant, or so that output light intensity of the optical amplifier 100 is kept constant (constant output control) or so that a signal gain of the optical amplifier 100 is kept constant (constant gain control).
The interrupting mechanism by detecting a loss of an input signal is described as below. A portion (a few percents) of an optical signal, which has been input via the input light connector 101, is branched by optical branch unit 102, and then photoelectrically converted by an input light detector 109. When photoelectric current detected by the input-light detector 109 becomes lower than or equal to a given level, a control circuit 110 judges that input light is lost. Accordingly, the control circuit 110 controls driving current of an pump wave source 111, which reduces or interrupts pump wave supplied to the optical amplification medium 103 to decrease a gain of the optical amplifier. The above-mentioned given level is generally set to a level such as an intermediate level between the minimum signal input level (that is determined by a signal-to-noise ratio of an optical signal and a configuration of a transmission system) and a noise level.
In addition, the optical amplifier, for which an input signal has been lost, outputs broadband noise light called ASE (Amplified Spontaneous Emission) instead of amplifying the signal light. The noise light may cause a bad effect including malfunction of: an input signal detecting circuit of an optical amplifier that follows; an amplifier with constant gain control; and an amplifier with constant output control. Because of it, the optical amplifier may be interrupted in order to avoid the bad effect.
The interrupting mechanism by detecting open of an output is described as below. An optical branch unit 104 is located at an output end of the optical amplifier 100. Optical signal 112 is branched from the optical branch unit 104 and is output to an optical fiber transmission line. Another branched light is introduced into an output-light detector 107. In addition to it, the optical branch unit 104 branches a portion (a few percents) of reflected light 113, which is input via the light connector 105, in reverse direction to introduce it into a reflected-light detector 108. Those signal intensities are photoelectrically converted and then are input to the control circuit 110. When a ratio of reflected light intensity to output light intensity exceeds a given level, the control circuit 110 judges that the output end is open. Accordingly, the control circuit 110 controls driving current of the pump wave source 111, which reduces or interrupts pump wave supplied to the optical amplification medium 103 to decrease a gain of the optical amplifier. The above-mentioned given level is generally set to a level intermediate between a Fresnel reflection level, which occurs when a connector is opened or a fiber is cut, and a reflection level of an optical signal caused by Rayleigh scattering produced in an optical fiber transmission line (in ordinary cases, the former is larger).